In sewage treatment plants, the incoming sewage, together with activated sludge, flows progressively and relatively slowly through aerating tanks. Pumps draw air from the atmosphere and force it through header pipes which carry the air along various paths beneath the surface of the liquor. Connectors secured to the headers perform the functions of supporting diffusers at longitudinally spaced points along the headers and passing air from the interiors of the headers to the diffusers. Air bubbles released from the diffusers aerate the liquor, promoting the metabolic processes of the activated sludge, and thus assist in digestion of the sewage.
An early example of connectors is found in U.S. Pat. No. 2,221,346 to A. C. Durdin, Jr. These are internally threaded bosses which project laterally from the header.
In the 1960's underslung headers were promoted and sold by the assignee of the present application. Holes were drilled at longitudinally spaced intervals along the otherwise underformed bottom wall of the header. Inverted cast tees were then welded to the outside of the headers with the bases of the tees in registry with the aforementioned holes. The internally threaded arms of the tees, which extended laterally, received the threaded ends of the diffusers.
In 1971, the present applicant filed an application for patent, Ser. No. 168,685 which was believed to disclose further improvements in connectors. FIGS. 3, 4 4A, 5, 6 and 7 herein are the same as in Ser. No. 168,685. In accordance with one embodiment, disclosed in FIGS. 3, 4 and 4A, transverse underslung connectors were provided with nipples drawn therefrom which were welded to nipples drawn from the bottoms of the headers.
However, according to Ser. No. 168,685, the connector arrangement which was preferred because of expected low resistance to the passage of air was that shown in FIGS. 5, 6 and 7. It was suggested that such connector be produced by making an oval opening in the top of a connector, making a matching oval opening in the bottom of a header, assembling the parts with the oval holes in registry and then welding them together from the outside around the peripheries of the holes.
Other considerations, not discussed in Ser. No. 168,685, reinforced the expectation that the FIGS. 5-7 embodiment would be superior. The oval hole cut in the connector provided a saddle of sorts. The connector, saddled across the header, did not project radially downward from the header to the same extent as the type with nipples (FIGS. 3, 4 and 4A) or the type with the welded tee. Thus, shocks against the arms of the connectors during transportation, handling and operation would exert less leverage on the welds in the FIGS. 5, 6 and 7 embodiment lessening the probability of weld or material failure. Also, the connector and header were in contact with, and welded to, one another through a greater radial angle, suggesting that each would thus do a better job of reinforcing one another than in the previous embodiments.
As was explained in Ser. No. 168,685, the alignment of diffusers in an aeration tank exerts a critical influence on liquor circulation, uniformity of air distribution and aeration efficiency. A 1 inch difference between the elevations of two diffusers on opposite sides of a header, measured at lateral distances of 2 feet from the sides of the header, can produce a very noticeable and significant difference in uniformity of air distribution, especially at low air flow rates, e.g., on the order of 10 scfm per diffuser. Calculations will show that such critical difference in diffuser elevations can occur when the axis of the diffuser diverges from the horizontal by only about 1.degree.. Thus, alignment of connectors is also critical.
Due to difficulties in matching the shape, position and alignment of the oval holes of the saddle type connector embodiment (FIGS. 5-7), and to provide a "land" for welding, it seemed necessary for purposes of achieving minimum acceptable production efficiency and economics to cut the oval opening in the connector smaller than the opening in the header. Consequently, the edge of the smaller opening in the connector protruded into the opening between the header and connector. The edge increased the friction effect of the opening upon the air passing from the header in the connector. Increased head losses were the result, and these can be very significant in the typical aeration system. For instance, for one actual municipal sewage treatment plant, the over-all head loss which could be attributed to the use of the saddle type connectors throughout was computed at about 4 inches of water. Based on a 2 cent per kwh power cost and the use of five 90,000 scfm centrifugal blowers to pump air through the headers, connectors and diffusers, the value of the wasted power was computed at more than $40,000 annually.
Thus, contrary to expectations, the saddle type of connector was not preferable. It suffered from very definite problems both in respect to the efficiency and economics of its production and in respect to the head losses and power wastage it caused.
Accordingly, it is an object of the present invention to provide improved aeration systems in sewage treatment tanks wherein the connectors are convenient to fabricate with the necessary degree of alignment accuracy to foster proper diffuser position and uniform air distribution. Another object is to provide systems which attain the foregoing object while minimizing head losses attributable to header-connector shape and position relationships. Still another object is to provide systems which attain the foregoing objects and nevertheless employ header-connector assemblies of adequate durability to retain their alignment during handling, transportation and use, even if the headers are fabricated of relatively thin wall metal, such as for instance 8 gauge and lighter stainless steel. One or more of these objects, and other objects which will be appreciated by those of ordinary skill in the art, will appear from the disclosure of the present invention which is provided by the accompanying drawings and specification.